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The influence of the wall model on the simulation calculation of heat transfer in the cylinder

when the diesel engine is working, the flow of the air flow in the cylinder will have a crucial impact on the wall heat transfer. Therefore, when the multi-dimensional transient simulation calculation of the heat transfer process in the cylinder is carried out, it is necessary to accurately simulate the gas flow in the cylinder first. However, because the gas flow in the cylinder has the characteristics of strong transient, strong compression, strong rotation and anisotropy, it is an extremely complex multi-dimensional turbulent motion. In order to carry out multi-dimensional transient simulation of the gas movement in the cylinder, a certain turbulence model must be used. At present, most turbulence models are based on the phase equilibrium of turbulent kinetic energy generation and dissipation, and are mainly applicable to the turbulent region at a certain distance from the wall. In the turbulent motion close to the wall, the turbulent fluctuation decreases due to the wall constraint, and the molecular viscous diffusion is gradually enhanced. Therefore, the diffusion term accounts for a large proportion near the wall, and the turbulent Reynolds number is very small, so the turbulence model cannot be directly applied to this region. In order to apply the existing turbulence model to the multi-dimensional numerical simulation in the cylinder, special treatment of the wall is needed. It can be seen that the treatment of different wall models near the wall will determine whether the multidimensional simulation calculation of the heat transfer process in the cylinder is accurate. Therefore, this paper uses the multi-dimensional transient simulation calculation method to investigate the influence of different wall models on the multi-dimensional transient simulation calculation of heat transfer in the cylinder. Through the simulation calculation, the best multi-dimensional transient simulation calculation model of heat transfer in the cylinder can be obtained, and then provide beneficial support for determining the multi-dimensional transient simulation of the working process in the cylinder

establishment of the model

1. turbulence model

in the whole working cycle, the gas in the cylinder of the diesel engine asks: the flow of the operator of the tensile testing machine is a complex structure including turbulent shear layer, boundary layer and reflux area, and always carries out extremely complex and strong transient unsteady three-dimensional turbulent motion. This turbulent motion is a common basis of various physical and chemical sub processes in the diesel engine operation, combustion process and heat transfer process. It determines the transportation and spatial distribution of various quantities in the cylinder, and has a direct and essential impact on the formation of combustible mixture and its concentration field distribution, flame propagation speed and combustion quality, heat transfer in the cylinder and pollutant generation. Therefore, to correctly analyze the working process, heat transfer, spray, combustion and emission of diesel engine in cylinder, it is absolutely inseparable from the correct description and Simulation of turbulent motion in cylinder. It is for this reason that the research of turbulence model has been widely developed in the simulation of internal combustion engine cylinder, and a large number of turbulence models with different structural forms have appeared

the transformation to the deep-seated strategic cooperation of technology, industry and talents in the combination of schools and regions. Although a large number of turbulence models have been fully developed and widely used, there are still great differences in the scope of use of different turbulence models, and different turbulence models need to be selected for different flow problems. The selection of turbulence model mainly depends on the physical mechanism of the simulated flow, the experience accumulation of similar problems, the accuracy requirements, computer resources and the time required to deal with the problems. In order to make an appropriate choice of turbulence models and give a reasonable analysis of the results of the strong mechanical design and development, electrical measurement and control, and software development capabilities of StarTech, we must understand the prediction capabilities and limitations of various models, especially for the numerical calculation of multi-dimensional transient simulation in diesel engine cylinder

in view of the importance of turbulence model to the whole simulation calculation, in order to ensure the calculation accuracy and realize fast calculation, the turbulence model used in this paper is K- ζ-? Turbulence model. This model is based on Durbin's ν 2-? Based on the model, in which the velocity scale ζ=ν 2/k, substitute ν 2。 After correction, K- ζ-? Model and Durbin's ν 2-? Compared with the model, it is more stable and the convergence is greatly improved

2. Wall model

theoretically K- ζ-? The turbulence model is based on the phase equilibrium of turbulent kinetic energy generation and dissipation, and is mainly applicable to the turbulent region at a certain distance from the wall. However, in the turbulent motion close to the wall, the turbulent pulsation decreases due to wall constraints, and the molecular viscous diffusion is gradually enhanced. Therefore, the diffusion term accounts for a large proportion near the wall, and the turbulent Reynolds number is very small, so K- ζ-? The turbulence model cannot be directly applied to this region. The experimental study of simple wall turbulence shows that the flow in the near wall region can be divided into three layers: the inner layer is a viscous bottom layer, the molecular viscosity controls the flow, and the average velocity increases linearly with the distance from the wall, which is also called a linear bottom layer; The outer layer is an isoreynolds stress layer with little influence of molecular viscosity. Turbulence controls the flow, and the average velocity is distributed in logarithmic ratio with the distance from the wall, which is called the wall ratio of turbulence; There is a transition layer in the middle, and the molecular viscous stress and Reynolds stress belong to the same order of magnitude. It can be seen that in order to- ζ-? When turbulence model is applied to multi-dimensional numerical simulation of diesel engine cylinder, special treatment of wall surface is needed. Among them, the wall treatment methods mainly include: Standard wall function method, double zone wall function method, composite wall function method and direct solution of low Reynolds number K- ε Model, etc. For directly solved low Reynolds number k- ε In terms of the model, it has high requirements for the calculation grid. In the multi-dimensional transient numerical simulation of the material deformation and residual stress caused by the expansion caused by humidity in the diesel engine 1, the calculation grid changes continuously with the rotation of the crankshaft, which makes it difficult to meet the low Reynolds number K- ε The requirements of the model can not be well applied to the multi-dimensional transient numerical simulation calculation in the cylinder. Therefore, three other wall treatment methods are usually used in diesel engine in cylinder simulation calculation:

(1) standard wall function method

the standard wall function method ignores the viscous bottom layer and transition layer close to the wall. Assuming that the node P in the first layer of lattice leaving the wall is located in the Reynolds stress layer, the relationship between the first layer of lattice and the wall characteristics is established through the wall function, which is a semi empirical equation, which is derived through experimental verification and similarity principle

where: K is Karman constant; Subscript p represents the center point of a layer of lattice close to the wall; E is a constant

(2) double zone wall function method

the two zone model divides the flow field into viscous influence zone and full turbulence zone. Based on Formula 2, K is used for the full turbulence zone with rey>200- ζ-? Model, for Rey

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